This invention relates to the production of chemimechanical pulps from wood or other lignocellulosic materials, such as chips, shavings and sawdust, with ultra high yields and with improved strength properties. More particularly, this invention relates to the production of such pulps by means of a three-stage process involving (1) treatment of the wood with sulfur dioxide, (2) sulfonation of the lignin in the wood, using aqueous sulfite and/or bisulfite solutions, followed by (3) mechanical defibering.
The pulp and paper and related industries use many processes to produce pulp from wood chips and other lignocellulosic materials. These processes can be classified, for purposes of discussion, into four groups, shown below with the representative yields:
Chemical Pulps--up to 60% yield PA1 Semichemical Pulps--60-80% yield PA1 Chemimechanical Pulps--80-95% yield PA1 Mechanical Pulps--at least 90% yield
The yield ranges shown are approximate only.
Chemical pulps are prepared by cooking the wood chips (or other lignocellulosic material) at elevated temperatures and pressures with various chemical agents which dissolve the lignin and some carbohydrate material to leave relatively pure cellulose fibers at the 40-45% yield level or cellulose plus some residual lignin at somewhat higher yield levels (45-55 percent).
Mechanical pulps, at the other extreme, use mechanical means such as grindstones to defiber logs or disc refiners to defiber wood chips into pulp. These processes use water for cooling and dilution purposes so that the approximately 5 percent of the wood substance that is water soluble is lost for a net yield of about 95 percent.
Chemical pulps have many advantages due to their cleanliness, high strength, and ease of bleaching, but they are expensive to produce due to the low yield. Solid and gaseous waste products from the pulping process give rise to many environmental problems.
Mechanical pulps are much cheaper to produce due to their high yield and constitute an efficient use of forest resources. Such processes offer no gaseous pollution and relatively little BOD.sub.5 (biochemical oxygen demand, five-day test) discharge compared to chemical pulps.
The semichemical and chemimechanical pulping processes fall midway between the chemical and mechanical processes in these respects.
The increasing world-wide demands for pulp, paper and other forest products and the decreasing availability of fiber are creating an increasing need for the use of higher yield pulps. The present invention produces a high yield pulp that can replace some types of chemical or semichemical pulp in many products.
It is known that the treatment of wood chips with relatively small amounts of sulfite and bisulfite,at near neutral pH, and under relatively mild cooking conditions (100.degree. C.-150.degree. C., for 2-15 minutes) produces a softening effect on the chips which makes them easier to defiber and generally produces a cleaner and better draining pulp than can be produced by mechanical means alone. See "Ultrahigh Yield NSCM Pulping," by C. A. Richardson, Tappi, Vol. 45, No. 12, pp. 139A-142A (1962); Richardson et al., "Supergroundwood from Aspen," Tappi, Vol. 48, No. 6, pp. 344-346 (1965); Chidester et al., "Chemimechanical Pulps from Various Softwoods and Hardwoods," Tappi, Vol. 43, No. 10, pp. 876-880 (1960); Uschmann U.S. Pat. No. 3,607,618; Aitken et al. U.S. Pat. No. 3,013,934; and Asplund et al. U.S. Pat. No. 3,558,428.
However, the pulps produced by such processes, while being superior to conventional mechanical pulps in terms of cleanliness and drainage properties, do not have sufficiently good physical properties to justify their increased cost of production relative to the conventional mechanical pulps.
Better properties can be achieved by cooking under more severe conditions such as increased temperatures in the 160.degree. C.-240.degree. C. range, but the strength improvement is always accompanied by a loss in yield. Instead of yields of over 90 percent, the yields are reduced to about 70-85 percent. See most of the above publications and patents and Richardson U.S. Pat. No. 2,962,412; Zimmerman U.S. Pat. No. 1,821,198; Cederquist U.S. Pat. No. 3,078,208; Asplund et al. U.S. Pat. No. 3,446,699; Von Hamzburg U.S. Pat. No. 2,949,395; Olson U.S. Pat. No. 3,003,909; and Rasch et al. U.S. Pat. No. 2,847,304.
The disadvantages of the foregoing prior art processes were overcome by the process of the copending application for United States Patent of Michael J. Ford et al., Ser. No. 838,837, filed Oct. 3, 1977, commonly assigned, now U.S. Pat No. 4,116,758. The process of that application provides a chemimechanical wood pulping process whereby the wood is first sulfonated to at least about 85 percent of the maximum level of sulfonation, in an aqueous sulfite/bisulfite solution having a pH of between about 6 and 8.5 at a temperature of between about 100.degree. C. to 150.degree. C. for a period of between about 10 and 90 minutes to provide a wood yield of at least about 90 percent by weight, and thereafter subjecting the wood to mechanical defibration. It was discovered in accordance with that process, to the surprise of those skilled in the art, that the higher the degree of sulfonation, the greater were the strength properties of the pulp obtainable, without any substantial loss in pulp yield. The best strength levels achieved in accordance with that process at yields of above 90 percent were obtained with sulfite cooking liquors having a concentration of about 120 grams per liter of sodium sulfite. However, concentrations of sodium sulfite in the order of at least about 80, and desirably at least about 90, grams per liter also provided advantageous results in accordance with that process.
It has now been discovered in accordance with the process of the present invention that high strength pulps can still be obtained in yields in excess of about 90 percent without requiring the high degree of sulfonation of the process of said Ford et al. application, or the use of such strong solutions of sodium sulfite/sodium bisulfite. These are important aspects of the present invention, since they permit the use of lesser amounts of chemicals in lower concentrations, which thereby reduces the effluent chemical materials which must be treated to avoid environmental problems. What is more, there is also a concomitant saving in chemical costs.
The process of the present invention employs as an important first step the treatment of the wood or other lignocellulosic material with sulfur dioxide gas or an aqueous solution of sulfur dioxide. Thereafter, the sulfur dioxide treated wood is subjected in a second step to a sulfonation treatment with a solution of sodium sulfite/sodium bisulfite, and finally, in a third step, to mechanical defibration.
The present inventors do not purport to be the first to employ an initial treatment of wood with sulfur dioxide gas or its aqueous solution to be followed by sulfonation with sodium sulfite solution followed by mechanical defibration. Marshall U.S. Pat. No. 3,393,122 discloses such a process. The process of that patent, however, has an entirely different goal, namely, improving the brightness of wood pulp, and is different from the present process. Pulp brightness is not a problem encountered in the process of the present invention. The process of the Marshall patent employs substantially lesser amounts of sulfur dioxide in the initial treatment of the wood. The highest amount of sulfur dioxide employed according to the Marshall patent is no more than 0.5 weight percent based on dry wood and in all other disclosures, including the examples, much lower amounts of sulfur dioxide, such as in the order of 0.005 percent to about 0.2 percent by weight of wood, are employed. Marshall thereafter stores the wood before subjecting it to presteaming treatment at atmospheric pressure, followed by a sulfonation treatment in a sodium sulfite solution, without disclosing the strength of the solution, to be followed by mechanical defibration. As will appear below, the conditions of the present process are quite different from those of the Marshall patent. Steaming the wood chips prior to cooking will drive off the sulfur dioxide. As found in accordance with the present invention, retention of the sulfur dioxide in the wood until sulfonation in sodium sulfite solution is important.
A publication by Kleinert et al., Tappi, Vol. 47, No. 10, pp. 605-609 (October 1964) entitled "Thermal coalescense of the Lignin Phase in Wood," discloses the treatment of wood with sulfur dioxide gas, usually in the amounts of about 0.1 percent based on wood as a pretreatment to be followed by conventional solely chemical cooking treatments. The only cooking treatments disclosed are the conventional solely chemical acid sulfite and kraft cooking treatments providing the traditional low pulp yields in the order of only between about 30 and 50 percent.
It is well known that the physical properties of wood pulps are strongly influenced by the flexibility of the individual fibers--which flexibility permits the fibers to be brought into closer contact with each other during the pressing stages of the paper-making process. This, in turn, leads to better bonding and improved strength. Natural wood fibers are rendered relatively inflexible by the presence of large amounts (20-30 percent by weight) of lignin which is a relatively rigid material at moderate temperatures (less than 100.degree. C.) Fiber flexibility is improved in conventional chemical or semichemical pulping processes by removing, chemically, at least part, and in some cases, nearly all of the lignin.
The present invention modifies the lignin sufficiently to produce a marked change in its physical and chemical properties, but not enough to render it soluble in water or in the cooking liquor; it is, therefore, not substantially removed from the wood fiber, and yields are consistent with those of purely mechanical pulps (90-95 percent).
It is, accordingly, an object of the present invention to provide a high yield chemimechanical pulping process for producing pulp from wood chips and other lignocellulosic materials, including shavings and sawdust.
It is another object of the invention to provide a three-step process for producing high yield chemimechanical pulp from wood chips whereby the chips are treated with sulfur dioxide gas or its aqueous solution for a short time followed by pulping in a solution of sodium sulfite/sodium bisulfite and thereafter defibering the chips by customary mechanical means to provide a pulp having excellent strength characteristics.
Other objects will be apparent to those skilled in the art from the present description.